Often apparatus of this kind have to be operated at locations where vibrations, such as acoustic vibrations, are present, like in production facilities for semiconductors, also known as ‘FAB’s. In such circumstances it is important to use enclosures to insulate the apparatus from its environment, to be able to operate these apparatus within their boundary conditions.
Consequently enclosures with a substantial rectangular configuration are known which are adapted to contain an apparatus sensitive for acoustic vibrations, the enclosure comprising walls and acoustic damping material located within the wall.
These prior art enclosures need to be voluminous and heavy to be able to effect a sufficient insulation. This appears from the thickness of the walls which is commonly between 50 mm and 100 mm. This thickness is however often insufficient to provide the desired acoustic insulation. Of course the enclosure could be built thicker, but this either leads to a smaller internal volume of the enclosure, leaving less space around the apparatus, which is awkward during the installation and servicing, or to a larger external volume of the enclosure, resulting in added use of floor space.
U.S. Pat. No. 4,362,222 discloses an enclosure with a substantial rectangular configuration, adapted to contain an apparatus sensitive to acoustic vibrations, the enclosure comprising walls and acoustic damping material located within the wall, wherein the acoustic damping material comprises at least one absorbing body of acoustic energy absorbing material having the shape of a parallelepiped located adjacent to an edge of the enclosure.
In this prior art structure the damping material his formed by slabs having only a limited thickness, coherent with that fact that only a limited damping of acoustic frequencies in the frequency range for which the human ear is sensible is aimed for.
It has appeared to the inventor that the acoustic vibrations most disturbing the processes in the apparatus within the enclosure are surprisingly caused by standing acoustic waves within the enclosure. In most cases these apparatus are particularly vulnerable for vibrations with frequencies in the range between 50 Hz and 1000 Hz, as caused by the nature of these apparatus. This frequency area of the vibrations to be avoided is rather different from the frequency area for which the human ear is in particular sensible. This discrepancy avoids that prior art insulating features known to be effective for protection of the human hearing can be simply adapted for this purpose.
Further DE-U-200 11 448 discloses a building wherein absorbing bodies are arranged suspended on horizontal lines allowing the bodies to be moved along these lines, allowing the vibration absorbing bodies to be located adjacent to the edge of a building.
This kind of standing acoustic waves within this specific frequency area is efficiently damped by an enclosure of the kind referred to above wherein the size of at least one side of the at least one absorbing body is substantially equal to ¼ of the inner size of the enclosure in the same direction.
The space required for the absorbing body is even further reduced if this body has a substantially rectangular shape and if the size of at least one side of the at least one absorbing body is substantially equal to ¼ of the inner size of the enclosure in the same direction. Another advantage of this feature is the fact that such rectangular bodies are easily available.
To minimize disturbance of the operation of the apparatus within the enclosure it is advantageous if the volume of the absorbing body is as small as possible and if it is concentrated in a single location. This is the case if the enclosure comprises only one absorbing body, that the body is located adjacent to a corner of the enclosure and that all three sizes of the absorbing body are substantially equal to ¼ of the relevant inner sizes of the enclosure in the same directions.
Disturbance to operations within the enclosure is even further reduced if the absorbing body is located at one of the upper corners of the enclosure.
Although other damping materials, like natural wool and fiber composites are not excluded, it has appeared that mineral wool is particularly advantageous as a damping material, as it has good absorption properties, it has a low weight and it is cheap.
It has appeared to inventor that especially mineral wool with a density of 10-100 kg/m3 leads to advantageous results.
Despite its advantageous properties, mineral wool and other fiber like materials suitable as absorbing materials may generate dust, which is not only unpleasant for humans in the enclosure, but which may also have a disastrous influence on the delicate apparatus present in the enclose and on the processes executed by them.
Therefore it is advantageous if the absorbing body is packed in an envelope of flexible material. This will keep any dust generated in the absorbing body within the envelope, so that the dust is not expelled. Of course the material of the envelope should be chosen carefully, so that the acoustic waves are properly transferred to the absorbing body and the waves are not reflected.